Is he looking for "stock" power steering? Those things are made from unobtainium, although most parts are interchangable between 1/2 to thru 1 ton models.
A knock off of the Chevy power steering is available at CPP here:
http://www.classicperform.com/power1.html
Other kit alternatives when staying with the I beam front axle include:
1. an 800 series 70's GM truck power steering box mounted on the outside of the frame forward of the front axle (I have this and don't like it - bump steer because of short drag link)
2. a Toyota 4x4 power steering box mounted where the original box was
3. a rack and pinion mounted to the back of the I beam
All three could use the original power steering pump available for the Pontiac V8. The mounting bracket for the pump is the same from 55-59 truck and 55-57 Pontiac car. 58 car bracket is somewhat different, although it still uses the same two holes on the top of the timing cover and would work OK in the truck. I got my pump and bracket from a '56 Pontiac. Found a rebuild kit (new seals and bearings - vanes not included, mine were OK) for the pump in New Jersey (the guy advertises in Hemmings) and rebuilt it myself.